Re: [PATCH] iommu/arm-smmu-v3: Simplify useless instructions in arm_smmu_cmdq_build_cmd()

[Robin Murphy <robin.murphy@arm.com>]

On 2021-08-17 12:34, Zhen Lei wrote:
> Although the parameter 'cmd' is always passed by a local array variable,
> and only this function modifies it, the compiler does not know this. The
> compiler almost always reads the value of cmd[i] from memory rather than
> directly using the value cached in the register. This generates many
> useless instruction operations and affects the performance to some extent.

Which compiler? GCC 4.9 does not make the same codegen decisions that 
GCC 10 does; Clang is different again. There are also various config 
options which affect a compiler's inlining/optimisation choices either 
directly or indirectly.

If it's something that newer compilers can get right anyway, then 
micro-optimising just for older ones might warrant a bit more justification.

> To guide the compiler for proper optimization, 'cmd' is defined as a local
> array variable, marked as register, and copied to the output parameter at
> a time when the function is returned.
> 
> The optimization effect can be viewed by running the "size arm-smmu-v3.o"
> command.
> 
> Before:
>     text    data     bss     dec     hex
>    27602    1348      56   29006    714e
> 
> After:
>     text    data     bss     dec     hex
>    27402    1348      56   28806    7086
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> ---
>   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 18 +++++++++++++++---
>   1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> index d76bbbde558b776..50a9db5bac466c7 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -233,11 +233,19 @@ static int queue_remove_raw(struct arm_smmu_queue *q, u64 *ent)
>   	return 0;
>   }
>   
> +#define arm_smmu_cmdq_copy_cmd(dst, src)	\
> +	do {					\
> +		dst[0] = src[0];		\
> +		dst[1] = src[1];		\
> +	} while (0)
> +
>   /* High-level queue accessors */
> -static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
> +static int arm_smmu_cmdq_build_cmd(u64 *out_cmd, struct arm_smmu_cmdq_ent *ent)
>   {
> -	memset(cmd, 0, 1 << CMDQ_ENT_SZ_SHIFT);
> -	cmd[0] |= FIELD_PREP(CMDQ_0_OP, ent->opcode);
> +	register u64 cmd[CMDQ_ENT_DWORDS];
> +
> +	cmd[0] = FIELD_PREP(CMDQ_0_OP, ent->opcode);
> +	cmd[1] = 0;
>   
>   	switch (ent->opcode) {
>   	case CMDQ_OP_TLBI_EL2_ALL:
> @@ -309,6 +317,7 @@ static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
>   		case PRI_RESP_SUCC:
>   			break;
>   		default:
> +			arm_smmu_cmdq_copy_cmd(out_cmd, cmd);

Why bother writing back a partial command when we're telling the caller 
it's invalid anyway?

>   			return -EINVAL;
>   		}
>   		cmd[1] |= FIELD_PREP(CMDQ_PRI_1_RESP, ent->pri.resp);
> @@ -329,9 +338,12 @@ static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
>   		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIATTR, ARM_SMMU_MEMATTR_OIWB);
>   		break;
>   	default:
> +		arm_smmu_cmdq_copy_cmd(out_cmd, cmd);

Ditto.

>   		return -ENOENT;
>   	}
>   
> +	arm_smmu_cmdq_copy_cmd(out_cmd, cmd);

...and then it would be simpler to open-code the assignment here.

I guess if you're really concerned with avoiding temporary commands 
being written back to the stack and reloaded, it might be worth 
experimenting with wrapping them in a struct which can be passed around 
by value - AAPCS64 allows passing a 16-byte composite type purely in 
registers.

Robin.

> +
>   	return 0;
>   }
>   
> 
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